An organic compound can take various structures compared with an inorganic compound, and it is possible to synthesize a material having various functions by appropriate molecular-design of an organic compound. Owing to these advantages, photo electronics and electronics, which employ a functional organic material, have been attracting attention in recent years.
A solar cell, a light-emitting element, an organic transistor, and the like can be exemplified as an electronic device using an organic compound as a functional organic material. These devices take advantage of electrical properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been making remarkable progress.
It is considered that the light emission mechanism of a light-emitting element is as follows: when a voltage is applied between a pair of electrodes which interpose a light-emitting layer, electrons injected from a cathode and holes injected from an anode are recombined in the light-emitting layer to form a molecular exciton, and energy is released to emit light when the molecular exciton relaxes to the ground state. As excited states, a singlet excited state and a triplet excited state are known, and light emission is considered to be possible through either of these excited states.
In an attempt to improve the performances of such a light-emitting element, there are many problems which depend on the material, and in order to solve these problems, improvement of the element structure and development of a material have been carried out.
For example, in Patent Document 1: United States Patent Application Laid-Open No. 2005-0260442, an anthracene derivative exhibiting green light emission is disclosed. However, in Patent Document 1, only the PL spectrum of the anthracene derivative is described, and the device performance is not disclosed when the anthracene derivative was applied to a light-emitting element.
Also, in Patent Document 2: Japanese Published Patent Application No. 2004-91334, a light-emitting element using an anthracene derivative as a charge transporting layer is mentioned. However, in Patent Document 2, there is no description on the lifetime of the light-emitting element.
If commercialization is considered, extending the lifetime is an important issue. Further, the development of light-emitting elements with much higher performances is desired.